Why is Rat Kiley's foot being shot off central to "The Things They Carried"?

It's important a) because it is true to life (many young soldiers really did this) and b) because it shows just how desperate the soldiers were. Here they were, trained members of the American military, and they were so unhappy with their situation and the war they were fighting that they were willing to shoot themselves to escape it. That's pretty bad. Since he also shoots a baby animal, not for food but just to hurt it, we can also assume that one is symbolic of the other: to live through Vietnam you have to hurt innocent things.